Abrasive composition and method of forming same



Patented Jan. 16,- 1945 ABRASIVE COMPOSITION AND METHOD or FORMING SAME Hermann Kott, West Orange, N. J., assignor to Fish-Schumann Corporation, New York, N. Y., a corporation of New York No Drawing. Application July 9, 1943, Serial No. 494,078

6 Claims.

\ This invention relates to abrasive compositions of matter and more particularly to abrasive and cutting tools wherein the abrasive material utilized therein consists of small particle sized diamonds and has for its object the provision of a method of producing such abrasive and cutting 1 tools.

Another object is to provide an improved abrasive or cutting tool wherein the abrasive material incorporated within the tool consists of small particle sized diamonds.

Still another object is to provide an improved grinding tool for use in the grinding of glass lens blanks.

Other objects will be apparent as the invention is more fully hereinafter disclosed.

In the fcrming of abrasive and cutting tools from small particle sized diamonds, for example, diamond fragments having a particle size passing 100 mesh, the major problem involved is to provide a cementing matrix to hold the plurality of diamond particles together which will tenaciously adhere to the diamond surface and at the same time provide strength and rigidity to the tool.

In co-pending application Serial No. 488,566, filed May 28, 1943, by Hermann Kott and Murray Yawitz, which application is assigned to the same assignee as the present application, there is described and claimed an abrasive or cutting tool and method of forming same, in which the surface of the diamonds are first surfaced or coated with a relatively thin film and strongly adherent layer of a metal of the platinum group and then bonded together with an iron-carbon alloy containing carbon within the range .45 to 1.0% by heattreating a mixture of the coated diamonds and the finely divided iron-carbon alloy compacted into the desired size, shape and configuration to temperatures within the range 725-800 C. at which the iron-carbon alloy becomes austenitic.

I have found that in substitution for the ironcarbon alloy of the composition of the above identified application, I may employ substantially pure nickel containing not over carbon. The substitution of nickel for the iron-carbon alloy of the composition of the said co-pending application is not a substitution of substantial equivalents, since it is well recognized that nickel exists in but a single phase and that carbon brings about no effect in nickel that is analogous to that which it produces in iron. (See Metals Handbook, 1936 edition, pages 1242-1243.)

Nickel containing carbon not over 30% is about as hard as soft steel and is acid and oxidationresistant in which respect it is superior to steel or iron-carbon alloys, particularly where aqueous coolants are employed during grinding or cuttin operations.

In the practice of the present invention, I prefer to employ nickel metal powder produced by the reduction of nickel oxide with hydrogen at temperatures approximating 300 C. or nickel metal powder produced by the thermal decomposition of nickel carbonyl at temperatures approximating 180 C.

The nickel metal powder produced by either of the two methods is crushed, if necessary, to pass about 400 mesh and is intimately mixed with the diamond fragments passing mesh which are surfaced with a platinum group metal such as rhodium, in the relative proportionsdesired, and

the mixture is compacted under light to heavy pressure depending upon the particular grinding or cutting tool to be formed and the compacted product is heated under conditions excluding the atmospheric gases oxygen and nitrogen and preferably under a positive pressure of a non-carburizing' reducing gas such as hydrogen or carbon monoxide, to a temperature above about 650 C. but not above about 800 C. for a time interval required to sinter the compacted mixture to the desired density product.

As one skilled in the art Will recognize the particular sintering time and temperature may be varied widely depending upon the compacting pressure employed, the relative proportions of metal-surfaced diamonds to nickel powder employed, the density product desired, and the size, shape and configuration of the sintered product.

As one specific example of the present invention, but not as a limitation thereof, the method employed in the forming of a grinding tool for the rough grinding and shaping of lens blanks consisting of glass will be described.

Lens grinding tools are annular in shape with one edge beveled off to provide a plane surface extending to the opposite edge lying at the desired angle to produce the desired concave or convex surface upon the glass lens blank with which the annular tool is rotatively contacted. The diameter of the tool may be varied widely depending upon the size of lens to be ground. The tool is usually mounted upon the end of a revolvable spindle mounted to be axially rotated and arranged to be brought into pressure engagement with the surface of a lens blank which is fixedly mounted with its plane surface lying in a plane transverse to the plane of spindle rotation.

A common size of lens grinding tool is one having an outside diameter of about 2 inches, an

inside diameter of about 1 inches and a height of about V2 inch. This size tool is adapted to grind concave or convex lenses as small as 2 inches diameter up to as large as 3 inches diameter.

In the production of this grinding tool in accordance with the present invention, the diamond fragments crushed to pass 100 mesh but not to pass 120 mesh are thoroughly cleaned of surface dirt and grease. and are freed of associated impurities by methods heretofore employed in the art and are provided with a firmly adherent relatively thin film surface coating of a platinumgroup metal, preferably rhodium, in accordance with the invention described and claimed in my Patent No. 2,103,623 of December 28, 1937.

In the forming of this surface coating of a platinum-group metal upon the surface of the diamond fragments, I employ apparatus similar in type to that illustrated in Fig. 9 of the said patent but employ in place of the arrangement including plates 20 and grid 9 an arrangement including a revolvable mesh cage within which a plurality of diamond fragments to b coated are disposed, the mesh of the cage being smaller than the particle siZe of the diamonds. The cage is revolved continuously during the electronic projection of the platinum-group metal through the meshes of the cage onto the surface of the tumbling diamonds and is continued until the surfaces of all of the diamond particles have been coated to a depth of at least l 10- millimeters, the projection voltage being adjusted under any given conditions of electronic bombardment employed, in accordance with the invention of said patent, to provide the desired degree of adherence of the electronically projected metal onto the diamond surface.

It is believed apparent that it is exceedingly difficult to define the exact degree of adherence of this film that is required for the purposes of the present invention. The function of the film is to provide a mechanically adherent bond between the film and diamond surface and an alloy bond between the platinum-group metal of the film and the sintered nickel, and the thickness of the film must be at least suflicient to inhibit diamond disintegration by carbon solution in the nickel. It is only by means of directional electronic projection in accordance with the invention-of my patent above identified that the mechanical adherence of the platinum-group metal onto the relatively smooth diamond surface is obtained. Surface coating by any of the other means known in the art, such as electro-deposition, thermal decomposition and by the projection of fused metal onto the surface by means of a blast of air or gas under pressure has been found to be not effective for the purpose of the present invention and usually provides merely a barrier film to diamond disintegration by carbon solution without the accompanying benefits of mechanical bonding necessary in abrasive compositions of matter falling within the scope of the present invention.

Hence, under any given conditions of electronic projection in accordance with the invention of my said patent, it will be necessary for one sk lled in the art to determine by experimentation the minimum voltage necessary to obtain the degree of mechanical adherence required of the film of electronically projected metal onto th surface of the diamond. This, of course, depends in part upon the diamond size and in part upon the particular irregularity of surface contour imparted to the diamond fragment by the crushing operation, as one skilled in the art will perceive. This experimentation, however, is believed well within the expected skill of one skilled in the art.

In the forming of the lens grinding tool in accordance with the present invention, the metalsurfaced diamond particles are mixed with the substantially pure nickel powder passing about 400 mesh, the relative proportions of diamonds to metal powder, preferably but not necessarily, approximating the ratio of one part diamonds to 10 parts nickel powder (by weight) and usually at least within the range 1 to 5 and 1 to 10.

The particular ratio of diamonds to nickel powder may vary widely without essential departure from the present invention, depending in part upon the grinding efllciency desired which is a function of the particle size and character of diamond surface as well as a function of the total number of diamond particles per unit surface area in the tool, and in part upon the amount of compacting pressure to be employed. In general, excessive compaction of the mixture is undesirable for two reasons. The first is that metal powders normally contain considerable amounts of surface adsorbed and occluded gases, which during sintering are evolved and where the compacting pressure is too high produce voids within the interior of the sintered metal product. The second reason is that with too high a pressure the metal powder is likely to be forced through the enveloping film of metal surfacing the diamond into contact with the diamond surface, thereby bringing about diamond disintegration by carbon solution in the nickel during the sintering operation.

Accordingly, I prefer to employ as low a pressure possible with any given mixture of diamonds and metal powder as will produce on heat-treatment at temperature approximating but below the thermal disintegrating temperature of the diamond (800-850 C., depending upon particle size) a sintered product of the density and tensile strength adequate for the particular field of use contemplated for the tool.

In the case of a lens grinding tool the density and tensile strength required for the tool is that which will resist the crushing pressure resulting from the pressure contact of the revolving tool against the glass surface. As this pressure normally is very light, a wide variation in density and tensile strength is adequate for the purpose.

I have found, therefore, that by compacting the 1 to 10 mixture of metal surfaced diamonds and nickel powder, under a pressure of from 10 to 20 pounds per square inch in an annular mold consisting of a refractory metal surfaced interiorly with an inert refractory such as aluminum oxide, adequate density and tensile strength is obtained in the sintered product for the purpose in view, by sintering the campacted material, without removing same from the mold, at a temperature of about 750 C. for a time interval of about 3 hours under a positive pressure of CO.

The nickel powder which is preferred in the practice of the present invention is that which is produced by the thermal decomposition of nickel carbonyl, particularly where the carbon content of this metal powder is below about .30%. A carbon content above .30% in the nickel powder is undesirable as on cooling to atmospheric temperatures the carbon in excess of the solubility at atmospheric temperatures precipitates out and lowers the strength of the sintered product.

Where the carbon content of the nickel powder vention generically and specifically and given one is in excess of .30% I may incorporate within the diamond-metal powder mixture a proportion of a thermally decomposable or carbon reducible metal oxide operative at the temperature of sintering to lower the carbon content of the sintering nickel Nickel oxide also may be employed with specific example of the practice of the same it is believed apparent that the same may be widely modified without essential departure therefrom and all such modifications and adaptations are contemplated as may fall within the scope of the following claims.

What I claim is:

l. The method of forming abrasive compositions of matter consisting of small particle sized diamond fragments dispersed throughout a nae-- tallic matrix which comprises forming a mixture consisting of small particle sized diamonds sunthe art that in place of rhodium I may employ any of the other platinum-group metals, as each of these metals are characterized by properties which render them substantially equivalents one for the other in the present invention. The most essential characteristic being that the surface film layer of the said metals are stable in air and upon heating to the temperature of sintering maintain clean metal surfaces adapted for ready alloying with the sintering nickel. ,Moreover, each has a relatively low solubility in nickel at the temperature oi sintering and a relatively low solubility for carbon. While the alloy bond; per se, that is formed between the platinum-group metals and the nickel may vary somewhat insofar a the physical properties of the bond is conearned, the bond obtained will be of the same order. The metal rhodium appears to form a most satisfactory bond under the conditions hereinabove described. However, any of the other platinum-group metals form intermetallic bonds of similar satisfactory characteristics.

Having hereinabove described the present infaced with a firmly adherent relatively thin film of a platinum-group metal and substantially pure nickel metal powder, compacting the mixture and,

heat-treating the compacted product to temperatures approximating but below 800-850" C. for an extended time interval adapted to sinter the mass to a coherent metal body.

2. The method of claim 1, wherein the said nickel powder contains carbon not exceeding about 30%.

3. The method of claim 1, wherein said platinum-group metal consists of rhodium.

4. An abrasive composition of matter consisting of a plurality of small particle sized diamonds distributed throughout a metallic mass, said composition being characterized by a relatively thin film of a platinum-group metal enclosing each said diamond particle, said iilm being mechanically bonded to the diamond face and being alloy bonded to the metallic matrix.

5. The composition ofclaim 4, wherein said platinum-group metal consists of rhodium.

6. The composition of claim 4, wherein said metallic matrix consists of substantially pure nickel containing not, over .30% carbon and said platinum-group metal consistsof rhodium.

HERMANN x o'r'r. 

